PL by bucket

[onenyala]

As a mental exercise I have been trying to fix the position of my landlocked house by sun sight.
The method I use is as follows:
In the absence of an horizon I use a bucket of water and view the sun through both the index glass and the reflected sun in the bucket in the horizon glass. I bring both images of the sun together so they are superimposed on each other.
This gives me an observed altitude x 2.
Of course I take the exact time of the observation.
I use a DR obtained by GPS
I then proceed as follows:

Observed double Altitude corrected for index error / 2
Correct this altitude for refraction.
Height of eye is not applicable as the eye is on the plane of the ‘horizon’
Semi-diameter not applicable as the images of the sun are superimposed on each other.
From the true altitude the TZD is then known.
Using the data from the N.A for the GMT of the observation and the GPS position as a DR I then calculate the CZD.
Comparison then gives me the intercept and I calculate the bearing.
As the house is fixed there I use the same DR for all of the observations.

After having taken numerous observations (50 – 60) over the course of the summer I get a position which is about 3 miles south of the GPS position??
Has anybody any explanation?

Stuart Jones

 

[golf]

Mental Exersize ? Beware the men in white coats !

 

[claymore]

Ah well, it all goes to show.
Have you tried a bucket of seawater or are you using fresh?
You may well find that the root of the problem lies here. However to be slightly more serious, If you are on passage across an ocean of thousands of miles and are only three miles off with your plotting I think you should be quite pleased, After all navigation is not an exact science, merely an estimate added to a guess to produce a supposition.
In the case of your house, does the 3 miles place you in a lower band for council tax and could you seek recompense from your local authority?

 

[Twister_Ken]

Tectonic Plate Drift? nm

§

 

[Twister_Ken]

Dozy Question - index error

As I haven't picked up a sextant for about 20 years all the data my brain used to hold in RAM about sights has become corrupted. But is it possible, because you are superimposing two images, that the index error is either cancelling itself out, or should be doubled?

 

[AndrewB]

Bucket & chuckit?

The reflection method is a not as easy as the usual horizon method. Even so, I am surprised you get a consistent error of 3 miles.

Your assumptions sound OK, without seeing the detail of the calculation. Presumably you are taking two daily observations to fix position?

Maybe a water surface is not a 'perfect' reflector, but adds some refraction? Maybe the sextant mirrors need adjusting, or worse, the sextant is warped?

 

[davel]

Perhaps it's telling you to move house?

 

[AndrewB]

Good thinking ...

... the IE correction must be made BEFORE halving the angle, whereas the refraction correction must be applied AFTER. But the way Onenyala explained it, that's what he did.

 

[onenyala]

I guess as you are having trouble spelling 'exercise' you wont be joining the technical discussion !
Stuart Jones

 

[onenyala]

Re: Bucket & chuckit?

Given that the water is still I cannot see why it should not be a perfectly level mirror. As the light does not pass through the surface of the water the refractive index of the water should not come into it.
I have taken numerous observations with the sun bearing from east round to west. A possible explanation is that the atmosphere is generally warmer in the afternoon and so could disproportionally alter the correction for refraction
So far as the sextant is concerned it is a heavy brass bodied one so should not be warped and the index error is small +/- 30 seconds.
Thanks for your response
Stuart jones

 

[onenyala]

Re: Dozy Question - index error

I do not think index error is a function of altitude.
Thanks
Stuart Jones

 

[onenyala]

the council tax idea sounds interesting
Thanks
Stuart Jones

 

[Bergman]

A few thoughts:-

Index error: Not entirely clear as to how you are doing the sum. I believe it should only be added (or subtracted ) once from the initial measured angle. The result of the calculation being divided by 2 to give altitude.

Is your location a significant height above sea level:- Could this affect the refraction figure - less atmosphere to refract the light.

If as you indicate the error is constant then are we looking at a datum issue. GPS will give whatever datum you have it set to. But what datum are your calculations based upon. Which I guess means which datum are the tables using. Don't know the answer to that one.

Fascinating problem, I'd love to know the answer.

Will you have to move house?

 

[Guest]

Re: Bucket & chuckit?

It's all the fault of the EC. The UK is gradually being towed south towards France as part of an EC wide harmonisation programme that has given us such startling developments as the UEB (the uniform euro bratwurst) and the multilingual toilet roll holder. It also minimises the distance between Downing street and Tuscany.
The previous answers assumed that the gps was correct. Probably is, but have you checked the datum. On the wgs datum, 0deg aparently no longer goes through Greenwich but through a local football field, and the Indian takeaway.

 

[bob_tyler]

Re: Bucket & chuckit?

Don't forget that from Jan 1 all transactions in Euros will, by EU edict, be called Eurines.

 

[AndrewB]

Sextant errors.

I've checked with a local tame physicist. You are right that there should be no perceptible error from using a reflection. However, temperature variations in atmospheric refraction cannot possibly be enough to account for this error, at normal altitudes.

You have checked index error (also that it holds steady, and there is no backlash on the micrometer) - have you done the other routine mirror adjustments?

The older, heavy brass models are particularly prone to warping. One accidental dropping is all it takes. So-called 'antique' sextants are generally useless for this reason. If yours was secondhand and not certificated when you bought it, that is a likely explanation of your problem.